1. Field of the Invention
The present invention relates to the production of precipitated silicas and, more particularly, to a novel process for producing synthetic precipitated silicas and silicates having new and improved physical and chemical properties.
2. Description of the Prior Art
As known in the art, finely divided amorphous precipitated silicic acid pigments and certain zeolitic type alumino silicates may be prepared by the acidulation of an aqueous silicate solution with an acid or a salt of the acid, such as aluminum sulfate. Such products are commercially available being sold, e.g., under the trademarks "Zeo"; "Zeolex"; and "Arogen" by the J. M. Huber Corporation. Specific examples of these products as well as methods for their preparation are disclosed in U.S. Pat. Nos. 2,739,073; 2,843,346; and 3,582,379.
Prior to the present invention, known and commercially available silicas were characterized by the following properties: high structure, high wet cake moisture content, high oil absorption, low valley abrasion, high surface area, and low pack density. In this regard, and due in part to the properties such as high oil absorption, high surface area, etc., the pigments have been widely and successfully used as reinforcing pigments in rubber.
However, the high wet cake moisture content is disadvantageous in that the drying and filtration rates are increased, thus increasing the overall cost in the production of the final product. For example, in known and conventional methods of producing silicic acid pigments or silicas, the wet cake moisture content of the product (following filtration of the precipitated reaction mass) is approximately 82%. This means that there can be recovered only 18 parts of dry pigment from 100 parts of wet cake.
In this regard and as noted above, there are a number of known techniques for preparing silica pigments which involve acidulating an aqueous silicate solution. Thus in U.S. Pat. No. 2,940,830 which issued June 14, 1960 to F. S. Thornhill, there is described a process for preparing finely divided silicas which are suitable as reinforcing agents in rubber compositions. Thornhill more specifically describes a process of preparing a silica material which is characterized by having an average ultimate particle size of 0.015 to 0.04 micron and a surface area of 25 to 200 square meters per gram by the controlled rate of addition of acid to an alkali metal silicate wherein the resultant slurry is constantly maintained at a pH above 7 in order to achieve the aforementioned end product characteristics. The Thornhill patent is specificaly directed to the production of a product suitable as a reinforcing agent in rubber compositions.
In U.S. Pat. No. 3,235,331 which issued Feb. 15, 1966 to Nauroth et al., there is described a process for producing a precipitated silica which is also stated to be useful as a reinforcing agent for rubber. More specifically, this patent discloses a process wherein an aqueous alkali metal silicate solution and acid are simultaneously added to a reaction vessel. In the Nauroth et al. patent, it is pointed out that this simultaneous addition is continued until the viscosity of the pool rises through a maximum and then falls to a substantially lower value. The amount of the acidification agent and the alkali metal silicate are proportioned so as to maintain the pH of the resulting slurry substantially constant throughout the major portion of the reaction and in the range of about 10 to 12. The process is generally conducted at a temperature of 80.degree. to 90.degree. C. and the end product, after drying, results in a silica which may have a surface area of 260 square meters per gram. The patentees point out that the product is satisfactory as a reinforcing agent for rubber.
In U.S. Pat. No. 3,445,189 issued May 20, 1969 to Matt et al., there is described a process for producing finely divided silicic acid by simultaneously adding solutions of an alkali silicate and a strong mineral acid to water at a temperature between 70.degree. C. and 90.degree. C. while maintaining the reaction pH between 7 and 9. The patentees point out that the product obtained by the aforementiond process is a finely divided nongelatinous silicic acid which is useful as a filler for natural and synthetic rubber and other elastomers. It is also disclosed in this patent that for a silica to be useful as a filler for natural and synthetic rubber and other elastomers, its surface area and oil absorption are of vital importance. This patent further discloses that extensive investigations have further indicated that if a finely divided silicic acid is to have good reinforcing properties for rubber, it must have a surface area of 100 to 250 m.sup.2 /g and an oil absorption of more than 2 cc/g or 200 cc/100 g. See column 2, lines 18 through 22.
In U.S. Pat. No. 3,730,749 which issued May 1, 1973 to James E. Morgan, there is disclosed a process for preparing silica for use in reinforcing compositions. It is pointed out in Morgan that the viscosity increase which occurs during the acidification or neutralization of aqueous alkali metal silicate is substantially minimized by adding a controlled amount of an alkali metal silicate. In Examples I, II, and III of this patent, it is also noted that the silica filter cakes had solid contents of 18.5; 24.9; and 25.1 percent, respectively. This means that the percent wet cake moisture of the silicas disclosed in Examples I, II, and III is one hundred minus the percent solid content in the filter cake. In other words, the percent wet cake moisture (% WCM) of silicas mentioned in Examples I, II, and III is 81.5; 75.1; and 74.9, respectively. The surface area, the average ultimate particle sizes, and rubber data of silicas produced by the teachings of Examples II and III are listed in Table 3 which also sets forth that rubber compositions incorporating the silicas of Examples II and III have desirable rubber properties. It is further substantiated by this patent that rubber properties of silicas are related to the wet cake moisture of the silica pigment. A silica of high percent wet cake moisture and suitable particle size and surface area has better rubber properties than the corresponding material of lower wet cake moisture. Thus the silicas disclosed in Morgan have a higher structure index, and therefore the silicas are useful rubber reinforcing fillers.
From the above it will be seen that the structure index of a silica is related to the rubber properties -- a silica of higher structure index will have better rubber properties than a silica of lower structure index. At this point, the various types of synthetic silicas, as well as "structure" and "structure index" should therefore be discussed.
In this regard, and as known in the art, commercially available synthetic silicas are derived either by a liquid phase or a vapor process. Silicas obtained by the vapor process are called fumed or pyrogenic silicas. Products obtained by the liquid process are categorized as silica gels and precipitated silicas. Thus, there are three distinct types of synthetic silicas on the market:
1. Pyrogenic Silicas
Pyrogenic or fumed silicas are prepared by reacting silicon tetrachloride vapor with oxygen and hydrogen gas at high temperatures. These products have high external surface areas and differ from other silicas (e.g., gels, precipitated silicas) prepared from the liquid phase process. Cabot and DeGussa are two suppliers of pyrogenic silicas.
2. Silica Gels
Silica gels are of two types -- hydrogels and aerogels. Hydrogels are prepared by reacting a soluble silicate such as sodium silicate with strong sulfuric acid. The gel is washed salt-free, dried, steam micronized, and then classified. Aerogels are prepared from crude hydrogels by displacing its water content with an alcohol. The alcohol is then recovered by heating the gel in an autoclave.
Aerogels are lighter and fluffier than hydrogels because the shrinkage of the gel structure is avoided during the drying process. Gels have very large surface areas, generally in the range of 300-1,000 m.sup.2 /g and high porosities. Silica gels are offered, e.g., by W. R. Grace and Company under the trademark "Syloid"; by Monsanto under the trademark "Santocel"; and Glidden under the trademark "Silcron."
3. Precipitated Silicas
Precipitated silicas are produced by the de-stabilization and precipitation of silica from soluble silicate by the addition of a mineral acid and/or acidic gases. The reactants thus include an alkali metal silicate and a mineral acid, such as sulfuric acid or an acidulating agent such as CO.sub.2.
When the acidification agent is added to the alkali metal silicate, at a certain point during the process, the silica starts precipitating. The addition of the acidification agent is continued until the M.sub.2 O of the alkali metal silicate (M being the alkali metal) of the ultimate silica is less than about 1% by weight. Thus, as a general rule, the acidification agent is added to the alkali metal silicate to neutralize the alkali portion bound to the silicate anion. The reaction slurry is filtered and washed free of reaction by-product, which is the alkali metal salt of the acidification agent. The filter cake is dried and milled to obtain a silica of desired degree of fineness.
Prior to the drying step, the silica filter cake generally results in a filter cake which contains a surprisingly high amount of water. For example, a silica which is useful as a filler for reinforcement of rubber and elastomers generally contains 80% to 85% water in its cake. For example, see Example No. I, U.S. Pat. No. 3,730,749 where the % wet cake moisture is 81.5. The percent water present in the filter cake is known as percent wet cake moisture or generally abbreviated as "% WCM." One hundred minus the % WCM gives the solid content of the filter cake, i.e., the amount of silica which can be recovered in the solid form upon drying the filter cake. The percent solid content of the filter cake is termed percent filter cake solids and generally abbreviated as "% FCS." Thus, % WCM and % FCS are related by the equation: EQU % WCM = 100 - % FCS
or EQU % FCS = 100 - % WCM
if we know the value of % WCM, we can calculate % FCS or vice versa.
Thus, a silica filter cake having 85% WCM will have 100 minus 85 of 15% FCS. This means that 15 pounds of silica can be recovered from such a filter cake by evaporating or drying 85 pounds of water from hundred pounds of filter cake. The total weight of filter cake consists of water and solid silica. In the example where the % WCM is 85, one can recover only 15 pounds of solid silica as can be seen below: EQU 100 pound filter cake = 85 pounds water + 15 pounds dry silica = 85% WCM + 15% FCS
thus, there are 85 pounds of water associated with 15 pounds of solid silica content of 85/15 .times. 100 = 567 pounds of water per 100 pounds of solid silica.
The water associated with the silica content of filter cake is structural water. This water is present whereby it occupies the available space between the silica aggregates and also the space inside the silica aggregates or micelles, micelles being defined by The American Heritage Dictionary of the English Language, College Edition, Copyright 1969, 1970, 1971, 1973, 1975, 1976; Houghton Mifflin Company, Page 828, as "a submicroscopic aggregation of molecules such as a droplet in a colloidal system; an organic particle of colloidal size found in coal; a coherent strand or structure in natural or synthetic fibers; a submicroscopic structural unit of protoplasm". As used herein, the term "structure" is defined as the ability of a silica to hold water in its wet cake. When silicas, such as the aforementioned known prior art products, hold a high percentage of water, i.e., from about 70 to 85%, they are known as high structure silicas. Materials holding less than 70% or from about 50 to 75% are referred to as low structure silicas. This total structural water content is a very important property of silica and is directly related to the functional and end use properties of silica. The amount of total structural water associated with 100 pounds of solid silica content of the filter cake is defined as "structure index" and abbreviated as S. I.
Mathematically, structural index (S. I.) of silica can be calculated if either the % wet cake moisture (WCM) or the % filter cake solid (FCS) values of said silica are known: ##EQU1## Structure index of silicas in wet cake moisture range of 80-85% is listed in Table I.
Table I ______________________________________ Structure Index of Silicas With % WCM of 85 - 80 % WCM 100 - % WCM S. I. ______________________________________ 85 15 567 84 16 525 83 17 488 82 18 455 81 19 426 80 20 400 ______________________________________
in addition to the above-discussed properties of known silicas, i.e., high wet cake moisture, structure index and high oil absorption, the low abrasiveness and high refractive index of known silica and silicate pigments renders them unsuitable for many uses. For example, it is well known that conventional synthetic precipitated silicas are unsuitable as polishing and abrasive agents in toothpaste compositions. See German Pat. No. 974,958; French Pat. No. 1,130,627; British Pat. No. 995,351; Swiss Pat. No. 280,671; and U.S. Pat. No. 3,250,680. In this regard, it is disclosed in U.S. Pat. No. 3,538,230 that known amorphous silicas such as precipitated silicas, pyrogenic silicas and aerogels are unsuitable for dentrifrice use because they show substantially no cleaning ability on human teeth because of their initial small particle size and because of the ease in which they break down into small particle sizes which result in poor cleaning ability.
Further, and in more detail, conventional silicas and amorphous precipitated alumino silicates, such as "Zeolex" and "Arogen," cannot be used for a clear gel toothpaste because of their high refractive index (1.55) and because they lack the needed abrasive and polishing characteristics when added to the toothpaste base composition. Clear gel toothpaste contains a high percentage of abrasive and polishing agent in the toothpaste formula. The major function of the abrasive and polishing agent is to remove stains, food debris, and bacterial plaque from the human tooth surface. Ideally the polishing agent should provide a maximum cleaning action at acceptable abrasion levels and must be compatible at high loadings of 15% up to 50% with other toothpaste formula ingredients. Thus known silicas and alumino silicates are unsuitable for clear gel toothpastes, (such as the product sold under the Trademark "Close-Up" by Lever Brothers) because they cannot be added at high loadings of 15% and above in a typical toothpaste composition. Because of their high oil absorption, high sorption characteristics and high refractive index (1.55) known precipitated pigments thicken up the dentifrice composition and impart undesirable opacity to the base paste resulting in an unacceptable product. In summary of the above, precipitated silicas and silicates cannot be used in conventional and clear gel dentifrice compositions because such products result in unacceptable toothpaste consistencies and do not possess the acceptable abrasive and polishing characteristics needed for use in dentifrice compositions.